Atomic frequency standards generate and maintain a standard frequency output by using the transition between two well-defined energy levels of an atom and the associated precise transition frequency to control the frequency of a frequency generating means. The atomic transition between two energy levels is employed as a highly stable frequency reference to which the frequency of a variable frequency oscillator, such as a voltage controlled oscillator can be electronically locked. The high stability and relative insensitivity to environmental perturbations that is associated with an atomic reference frequency is transferred to the variable frequency oscillator.
Hydrogen, cesium and rubidium frequency standards have been used to provide atomic controlled oscillators and standard frequencies, usually 5 megahertz (MHz) or 10 MHz. Such frequency standards have usually employed a voltage controlled oscillator, a physics package and associated electronics in an effort to maintain an accurate and stable standard frequency on a long-term basis. The physics package and associated electronics have been used to slave the voltage controlled oscillator to the frequency of the atomic transition, thereby reducing the drifting due to aging and environmental effects.
As well known in the art, physics packages have included resonator assemblies formed with a microwave cavity tuned to the atomic transition frequency, a closed transparent cell of atomic gas in the microwave cavity, means to inject microwave energy developed from the frequency of the voltage controlled oscillator into the microwave cavity, and a photodetector in the microwave cavity. An atomic light source in the physics package directs light into the microwave cavity of the resonator assembly to excite atoms of the atomic gas in the cell, and the light detector in the microwave cavity senses the intensity of light from the atomic light source that transits the atomic gas.
In such atomic frequency standards, atoms in the resonator assembly undergo hyperfine transition within a very narrow range of frequencies. For example, the hyperfine transition frequency, frequently referred to as the "atomic transition frequency" of rubidium (Rb 87) gas is nominally 6,834,687,500 Hz (6.834 . . . GHz) and a typical physics package has an atomic bandwidth of about 800 to about 2000 Hz. In order to lock the voltage controlled oscillator to the atomic transition frequency with the resonator assembly and to maintain a substantially unvarying standard frequency with the atomic transition frequency, a frequency-modulated microwave interrogation frequency is synthesized from the output frequency of the voltage controlled oscillator and is injected into the microwave cavity of the resonator assembly. The atomic gas within the transparent cell of the resonator assembly is excited by the incident light, the intensity of which is sensed by a photodetector to provide a physics package electrical output signal. The energy of the frequency-modulated microwave interrogation frequency injected into the microwave cavity interacts with the atomic gas within the transparent cell and varies the intensity of the light passing through the atomic gas in a manner dependent upon the difference between the injected microwave interrogation frequency and the atomic transition frequency, and the variation in light intensity is detected by the photodetector. The physics package thus provides a frequency discriminating electrical output signal that can be used to control the frequency of the voltage controlled oscillator.
Frequency modulation of an interrogation microwave frequency, within the atomic bandwidth of the atomic transition frequency, is used to determine if the interrogation microwave carrier frequency (which is a function of the standard frequency) is higher or lower than the atomic transition frequency, and a corrective signal is generated to correct the frequency of the voltage controlled oscillator and to lock it to the atomic transition frequency.
The resonator assembly of the physics package is thus the heart of an atomic frequency standard, and substantial efforts have been directed by workers in the field to modify atomic frequency standard physics packages and resonator assemblies to improve their operating characteristics, to reduce their size and cost, and to improve their manufacturability without deleteriously affecting their operation.
The resonator assembly of one such physics package being manufactured and sold by Datum/Efratom Time and Frequency Products, Inc., is shown in FIGS. 1 and 2. The resonator assembly of the Efratom physics package includes a microwave cavity 100 defined by a body 100a, encompassing an absorption cell 110, and a cavity lid 100b, which carries a microwave loop 120, a tuning screw 130 for the microwave loop, and a light detector 140. The lid 100b for the microwave cavity 100 is usually tilted in forming the microwave cavity, as shown in FIG. 2, to tune the microwave cavity 100 for proper operation. The microwave loop 120 consists of a series combination of a feed-through capacitor 120a, a carbon resister 120b, step-recovery diode 120c, and the portion 120d of the lid 100b forming the microwave cavity 100. An approximate submultiple of the atomic transition frequency is supplied to the microwave loop 120 through the feed-through capacitor 120a by a microwave cable (not shown) from the frequency generator of the atomic frequency standard.
In manufacturing the microwave cavity 100 shown in FIGS. 1 and 2, the microwave loop 120 and microwave cavity 100 are coarsely tuned by moving the lid 100b inside the microwave cavity body 100a, and fine tuning of the microwave loop 120 is accomplished by adjustment of the tuning screw 130. In such physics packages, the width of the microwave loop is generally greater than its height above the ground plane (i.e., the internal surface of the lid 100b), such microwave loops being typically about 0.290 inch wide and about 0.200 inch high. The advantage of this loop is that it also functions as a microwave multiplier and mixer using only three electronic components: a step recovery diode 120c, a capacitive feed-through 120a and a resistor 120b. It would be very difficult indeed to make a simpler or less expensive device that performs all of these functions simultaneously.
U.S. Pat. No. 4,405,905 to Busca, et al. discloses, as illustrated by FIG. 2 of Busca, et al., a physics package for an atomic frequency standard including excitation means comprising a microwave loop 5 which encircles an absorption cell 4 within the physics package to decrease the time constant of the physics package by broadening the width of the microwave absorption line of the absorption cell. The absorption cell 4 of Busca's disclosure has no buffer gas and includes a special absorption cell wall coating providing a "non-disorienting" layer. Busca, et al. state that the use of the cell-encircling microwave loop 5 permits the elimination of a microwave cavity and makes it possible to substantially reduce the volume of the absorption cell and to eliminate the necessity to tune the cavity and cell in the production of atomic frequency standards. The elimination of the microwave cavity is said to provide a variable phase (traveling wave) field in the absorption cell as contrasted with the constant phase (standing wave) field obtained with the use of microwave cavities.